Where an elevator car has hitch load weighing, it is suspended with ropes which pass between two crosshead beams and through holes in a hitch plate. The cab sits on a bottom beam connected to two side beams which are in turn connected to the crosshead beams. When passengers step into the cab, the tension in the ropes increases and causes the hitch plate to bear against the crosshead beams. A single load cell may be sandwiched between the hitch plate and crosshead beams to perform load weighing. Such a hitch load weighing assembly is shown in copending application, Ser. No. 07/792,978, filed Nov. 5, 1991. There, a load cell provides an analog load signal to an analog-to-digital converter (ADC) for converting an analog load voltage into a digital voltage for use by an elevator computer.
Tare is the weight of an empty container that is subtracted from the gross to obtain the net. A problem with hitch load weighing is that the tare is dynamic and varies as a function of car position due to the increasing or decreasing weight of the traveling cable and compensation ropes as the car goes up or down, and the tare at the bottom floor (the static tare) is not the same as the tare at the top floor. The difference in tare from the bottom to the top of the hoistway can be large relative to the weight of the car and the weight of the occupants. To compensate tare, one solution is to convert the analog load signal into a digital signal and compensate the digital signal for tare. This is shown in U.S. Pat. Nos. 4,181,946, "Digital Scale", and 4,630,696, "Apparatus and Method for Automatic System Calibration to Provide Enhanced Resolution in Computerized Weighing Systems".